This section introduces aspects that may be helpful to facilitating a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Fiber optic communications commonly use single-mode fiber (SMF), which offered substantially greater bandwidth than traditional twisted-pair copper channels. SMF allows only one spatial mode, or “fiber-mode,” of light to propagate through the channel, although multiple wavelengths (i.e., frequencies) may propagate within that mode. A mode defines how a wave is distributed in space and how it travels through space. Modes are described by the Helmholtz equations for waves; more specifically, each fiber-mode represents a particular solution to those equations. A SMF therefore exhibits little modal dispersion, making it suitable for long distance transmissions that require high fidelity and high bandwidth. Given their long distance and high bandwidth capabilities, SMFs are common among service providers, such as telephone companies and cable companies, and colleges and universities.
Alternate fibers which support more than one spatial mode have been developed including multi-core fiber (MCF) and multi-mode fiber (MMF). MCFs are fibers that have more than one core inside and allow multiple parallel spatial channels to provided additional capacity. Relative to SMF, MMF typically includes a single core that is larger than that of a SMF operating at the same wavelength, thereby allowing propagation of multiple modes through the core. Typically the larger core has been used to relax the accuracy of alignment between fibers and devices which may lend itself to lower-cost electronics and hardware for terminations and in-line equipment, making it suitable for applications nearer to the end users, MMF is limited by modal dispersion and chromatic dispersion. The impacts of distortions such as modal dispersion and chromatic dispersion are compounded over distance MMFs are commonly used as backbones for shorter distance local area networks (LANs) in buildings and on campuses.
To further improve the capacity of fiber systems, modulation techniques were developed to take advantage of the various dimensions of optical signals. One common technique is wavelength-division multiplexing (WDM), which utilizes two or more wavelengths to carry data. Another is polarization-division multiplexing (PDM), which utilizes differently, e.g. orthogonally, polarized signals to carry data. PDM is often combined with phase or quadrature modulation, which uses phase differences to distinguish signals. Yet another technique is space-division multiplexing (SDM) using MCF or MMF. SDM uses a spatial mode multiplexer (S-MUX) to launch multiple signals, into a MCF or MMF, and a spatial mode demultiplexer (S-DMUX) to demultiplex the multiple signals and coherently receive them.